1. Technical Field of the Invention
The present invention relates to a multiple access communication system such as, but not limited to, a CATV network, or a wide area network (WAN).
2. Description of the Prior Art
Conventionally, a WAN such as a satellite communication system, or cable modem system holds communication medium in common, and a key station and a plurality of user stations communicate with each other via this medium. Control signals and data signals from the key station are broadcasted on a broadcast type communication channel to all of the user stations, while data signals from user stations to the key station are transmitted on multiple access type channel by time division multiple access (TDMA) scheme. In general, a WAN has several hundreds or several thousands of user stations, but much fewer user stations are transmitting data simultaneously. Therefore, so-called demand-assign-multiple-access scheme which allows only the user stations demanding data transmission to use the communication channel is efficient in WAN. Well-known demand-assign-multiple-access schemes are random reservation scheme and random access scheme. By random reservation scheme, the key station allocates bandwidth on multiple-access type channel for reservation. User stations demanding data transmission send reservation packets in the allocated bandwidth for reservation. When reservation packets collide, the key station resolves the collision. The key station allocates bandwidth according to the reservation. Then user stations which have won the bandwidth transmit data packets without collision. By random access scheme, the key station allocates bandwidth on multiple-access type channel for data transmission, user stations demanding data transmission send data packets in the allocated bandwidth for data. When data packets collide, the key station resolves the collision.
However, the conventional demand-assign-multiple-access schemes as explained above have following disadvantages. Random reservation scheme achieves good throughput under highly loaded conditions, but has a disadvantage that even under collisionless conditions, packets suffer delay due to reservation process. By random access scheme, delay of data packet is minimum, when any collision does not occur. But, loss of bandwidth is large, when data packets collide. Therefore, in highly loaded conditions, the probability of collision rises and the throughput degrades.
Further, in such a multiple-access communication system that the bandwidth for data from user stations are allocated by the key station according to reservation from user stations, the user stations may not be treated fairly, when lengths of data packets are different and bandwidths are allocated in order of reception of reservation by the key station. Therefore, in order to achieve the fairness among the user stations, the so-called weighted fair queueing is employed as the bandwidth allocation scheme which schedules data transmission from the user stations on the basis of the reception time of reservation and the packet length of reservation, as disclosed in xe2x80x9cService Disciplines for Guaranteed Performance Service in Packet-switching Networksxe2x80x9d by Zhan, Proceedings of the IEEE, vol. 83, No. 10, October 1995, pp 1374-1396. However, these algorithms such as the weighted fair queueing increases a load on CPU, when reservations from the user stations come frequently, which is the case in such a system that includes great many user stations.
Therefore, an object of the present invention is to obtain high throughput by using the random reservation scheme only, when a lot of data is transmitted by user stations. Further, another object of the present invention is to suppress delay due to reservation process by using random access scheme in addition to random reservation scheme, when the traffic is low.
In the multiple access communication system of the present invention, the key station has means to allocate bandwidth on multiple access channel for user stations to send reservation packets in random reservation scheme, means to allocate bandwidth on multiple access channel for user stations to send data packets in random access scheme, and means to allocate bandwidth on multiple access channel for user stations to send data packets according to the reservation received by the key station. Further, the key station monitors traffic on the multiple access channel in order to control the bandwidth on multiple access channel for user stations to send data in random access scheme. Concretely, when the traffic is low, more bandwidth for user data in random access scheme is allocated. On the contrary, when the traffic becomes heavy, only the random reservation scheme is employed in order to assure transmission efficiency of the multiple access channel.
Further, the user station of the present invention transmits reservation information in bandwidth for random reservation and data packets in bandwidth for random access, by solving collisions on the random reservation system and random access system.
According to the present invention, when traffic on the multiple access channel is light, user stations are able to transmit directly data packets under shorter delay by using random access scheme, rather than using random reservation scheme which suffers unneglectable delay due to reservation and allocation of bandwidth for reserved data packets. Further, according to the present invention, when traffic on the multiple access channel is heavy and collisions occur frequently, the user stations are able to employ random reservation scheme to utilize the multiple access channel efficiently, rather than random access scheme in which loss of bandwidth by collision of data packets is relatively larger than that of reservation packets.
Further, if once allocated bandwidth for data packet, the user station can attach reservation information of successive transmission to the data packet. Therefore, the key station can collect information about the demand of the data transmission and can allocate bandwidth for data packet as soon as possible. Further, in order to accept reservation information from the user stations, even when traffic on multiple access channel is heavy, the key station allocates at least a certain amount of bandwidth for random reservation packets.
Furthermore, according to the present invention, user stations are treated fairly regardless of length of packets which they want to transmit, because bandwidth for short packet transmission is allocated with priority and at the same time minimum bandwidth is also guaranteed for long packet.